Previously, there has been disclosed various kinds of joining agents, such as inorganic solder comprising various oxides, fluorides and various metal solders comprising various metals for joining ceramic bodies to each other. For example, a joining agent composed of powder mixture of CaF.sub.2 and kaolinite is applied to parts to be joined of silicon nitride ceramic plates and then heated at 1400.degree. to 1500.degree. C. in furnace to obtain jointed ceramics by a reaction of the joining agent and the silicon nitride ceramic (Japanese patent application, 32084/1983). The use of the CaF.sub.2 is in a purpose that the CaF.sub.2 decomposes thermally at a high temperature and produces F.sub.2 gas which corrodes or eats away the surface of the ceramic. As a result, the reaction product between calcium and kaolinite promotes the reaction of the joining agent with the ceramics. When the joining agent is selected to be in a good wetability and a high reactivity with the ceramic, the joint part between the ceramic bodies are completed upon heating the joining agent at a temperature higher than its melting point. A conventional heating method is to heat the whole of ceramics with the joining agent in the furnace at an joining temperature to achieve the joining. When the ceramic body in a large size or a long size is jointed, it is necessary to use a large furnace or to take a long time for heating up or cooling down. This results in the problem such as the limitation of the equipment and an increase in the equipment cost or running cost. Further, the higher joining temperature may cause the thermal degradation of the ceramic body. Therefore, the problem occurs with the application field requiring the thermal resistance. Since the heating method to heat the whole of the ceramic body with the joining agent has various problems as mentioned above, it is desirable to achieve an joining method to heat partially only the part to be jointed. In order to solve the problems, U.S. Pat. No. 4,724,020 has reported one of electric joining methods for the ceramic bodies.
The electric joining method is carried out in the following way: The planes to be jointed of ceramic bodies are applied with an joining agent having an electric conductivity at a high temperature and are pre-heated by a gas flame to a temperature more than about 800.degree. C. in advance. A high voltage is applied across a pair of electrodes to supply electric current to the joining agent. Then the joining agent is heated by the Joule heat and is melted to complete the ceramic joining through the reaction between the joining agent and the ceramics.